1. Field of the Invention
The present invention relates generally to digital optical communication used for electric home appliances or information equipment having an infrared communication function, and more particularly, to a digital optical transmission apparatus and a method for performing ASK modulation to alleviate mutual interferences.
2. Description of the Background Art
In optical communication in general, the amplitude, phase or frequency of a subcarrier is usually changed for transmission depending upon data desired to be communicated. This is called carrier bank modulation as compared to modulation not utilizing a subcarrier (base band modulation). The subcarrier is created artificially by turning on/off light in a certain cycle. The subcarrier is often replaced with a square wave of light simply turned on/off.
Among carrier band modulation techniques, the simplest technique changes the amplitude. The technique is called ASK (amplitude-shift keying). Among various ASK techniques, the simplest one utilizes two kinds of amplitudes, a prescribed amplitude and an amplitude zero. The technique is more particularly called OOK (on/off keying).
A signal modulated according to base band modulation techniques such as RZ (Return to Zero) modulation, PPM (Pulse Position Modulation) and Manchester modulation may be superposed by a subcarrier for transmission. These modulation techniques are also ASK modulation techniques in a board sense. The waveforms according to these modulation techniques are given in FIG. 1. The modulation technique which changes the phase and frequency of a subcarrier are called PSK (Phase Shift Keying) modulation and FSK (Frequency Shift Keying) modulation, respectively. A waveform produced by PSK modulation and a waveform produced by FSK modulation are shown in FIG. 2.
Spectrum According to Conventional Carrier Band Modulation
A spectrum according to carrier band modulation has a main lobe in a frequency band around the frequency of the subcarrier. According to a technique using a plurality of subcarriers such as FSK, there are a plurality of main lobes in frequency bands around the frequencies of the subcarriers. One of the side bands of a main lobe according to a carrier band modulation technique is the inverse of the minimum xe2x80x9csubcarrier unchanged timexe2x80x9d normally used according to the modulation technique.
Spectrum According to Conventional Carrier Band Modulation Technique, Particularly ASK Modulation Technique
A spectrum according to an ASK modulation technique produced by superposing a waveform according to a base band modulation technique by a subcarrier corresponds to a spectrum produced by shifting the original spectrum according to a base band modulation technique to a frequency band around the frequency of the subcarrier. Note, however, the entire spectrum according to the base band modulation is not shifted to a high frequency band, and a part of the spectrum according to the base band modulation remains in the low frequency band as unwanted radiation. A spectrum according to an ASK modulation technique produced by superposing the waveform of an NRZ (Non Return to Zero) modulation technique by a subcarrier is given in FIG. 3.
Spectra According to Conventional Carrier Band Modulation Technique, Particularly According to PSK and FSK Modulation Techniques
According to PSK and FSK modulation, unlike the ASK modulation technique, no spectrum appears in a low frequency band. According to these techniques, however, more power is generally consumed than the ASK modulation and the circuit configuration of a receiver for the modulation technique is more complicated. As a result, ASK modulation techniques or base band modulation techniques are more often used in the optical communication industry.
As described above, according to the base band and ASK modulation techniques, a spectrum having a main lobe of a bandwidth of at least a bit rate appears in a low frequency band. Therefore, if there are transmitter/receivers according to a plurality of communication techniques, disadvantageous mutual interferences are caused among these transmitter/receivers.
A remote control used for a television, for example, employs an ASK communication technique at a bit rate of 1 Kbps using a subcarrier in the vicinity of 40 KHz. The spectrum has a main lobe of about 2 KHz around the vicinity of 40 KHz on one side. Herein, if a communication at about 75 Kbps is newly performed according to another communication method, a spectrum appears in a low frequency band about in the range from 0 Hz to 75 KHz whether the base band modulation or ASK modulation technique is employed. Therefore, optical communication using the remote control will be interfered with. Thus, while having a communication at about 75 Kbps according to the conventional base band modulation or ASK modulation technique, the interference between that communication and the remote control using a subcarrier at about 40 KHz has been hardly eliminated.
Spectrum According to IrBUS Method as Related Art
In order to solve the above problem, a 16 PSM (Pulse Sequence Modulation) coding method has been proposed in the IrBUS method that the United States Infrared Data Association (IrDA) is presently trying to standardize as a coding method to replace conventional base band modulation techniques. In the proposed coding method, as shown in Table 1, 4-bit data is allocated to 16 symbols represented by 8 slots.
In the IrBUS method, this 16 PSM base band signal is superposed by a subcarrier at 1.5 MHz to produce a power spectrum valley in the range 36 KHz to 40 KHz, a peak power spectrum band for a subcarrier normally used by a remote control for electric home appliances, such that mutual interference is alleviated. The use of this method may restrain mutual interferences to about xc2xd the level of the conventional base band or ASK modulation method.
FIGS. 4A to 4C show an optical signal produced by superposing a 16 PSM base band signal by a subcarrier, its subcarrier component and its base band component (details of which will be described later), respectively.
FIGS. 5A and 5B show a conventional LED driving circuit. The LED driving circuit shown in FIGS. 5A and 5B includes an LED current limiting resistor 105, a transistor 107 having its output controlled by an IrTx electrical signal 800, and an LED 106 which emits light when transistor 107 is turned on and outputs an IrTx optical signal 801. Note that the LED driving circuit shown FIG. 5A has LED current limiting resistor 105 between a power supply 104 and LED 106, and transistor 107 has its collector and emitter connected to LED 106 and ground, respectively. The LED driving circuit shown in FIG. 5B has LED 106 between power supply 104 and LED current limiting resistor 105, and transistor 107 has its collector and emitter connected to LED current limiting resistor 105 and ground, respectively.
When IrTx electrical signal 800 attains a high level, transistor 108 is turned on, which allows a current to be passed to LED 106 through LED current limiting resistor 105, and LED 106 emits light which is output as IrTx optical signal 801. When IrTx electrical signal 800 attains a low level, transistor 107 is turned off, so that no current is passed to LED 106, which therefore does not emit light.
FIGS. 6A and 6B show a power spectrum in the range from 0 to 2 MHz and a power spectrum in the range from 0 to 50 KHz, respectively when a 16 PSM base band signal superposed by a subcarrier is transmitted.
FIGS. 7A and 7B show power spectra shown in FIGS. 6A and 6B which are divided into its subcarrier component and base band component, respectively.
The component forming an optical signal waveform according to the IrBUS method and a power spectrum generated by the component will be now described.
A subcarrier in the case of an electric wave is an AC component having an amplitude in positive and negative directions around 0 V. In the case of an optical signal, however, a subcarrier is artificially generated by turning on/off light unlike the case of the electric wave, and therefore the subcarrier has an amplitude only in the positive direction. A subcarrier for an optical signal having such an amplitude A may be regarded as a signal produced by combining a DC current having an amplitude of A/2 and a sine wave (or a square wave) having an amplitude of A/2. Herein, the former is defined as the DC component of the subcarrier and the latter as the subcarrier component.
A carrier band-modulated optical signal may be divided into a DC component and a subcarrier component on the basis of a symbol defined according to the modulation method. Herein, a waveform produced by combining in time series the subcarrier components of subcarrier symbols included in the entire optical signal is defined as the subcarrier component of the optical signal. Similarly, a waveform produced by combining in time series the DC component of each of the subcarrier symbols included in the entire optical signal is defined as the base band component of the optical signal.
More specifically, when the bias levels of the DC components of the subcarrier symbols are all equal, the condition is expressed as xe2x80x9cthe DC component of the base band component of the optical signal is constantxe2x80x9d.
When the waveform according to the IrBUS method shown in FIG. 4A is analyzed for the above-described component, the waveform may be divided into the subcarrier component at 1.5 MHz and the base band component as shown in FIGS. 4B and 4C, respectively. Power spectra generated by the subcarrier component at 1.5 MHz and the base band component are as shown in FIGS. 6A and 6B, respectively.
The base band component shown in FIG. 4C is a base band signal according to the 16 PSM coding method before superposing the subcarrier, and the spectrum of the signal component will be unwanted radiation remaining in the low frequency band as described above.
In the 16 PSM coding method, such unwanted radiation in the central frequency band used by a remote control is restrained. However, the light receiving element of the remote control has a very high sensitivity and has a certain degree of sensitivity to a frequency apart from the central frequency. As a result, disadvantageous interference with the remote control cannot be completely solved even according to the 16 PSM coding method. The IrBUS method is a kind of ASK modulation, and the disadvantage is encountered as long as the conventional of ASK modulated signal is used.
Meanwhile, according to the FSK or PSK modulation method in which a subcarrier continues to be transmitted, the DC level of the base band component is constant, and therefore the above-described disadvantage is not observed. However, in optical communications in practice, data is divided into predetermined amounts and formed into packets for communication. For example, a plurality of such packets are communicated at a cycle of 13.8 ms according to the IrBUS method.
FIG. 8A is a waveform chart showing a waveform when packets according to the FSK modulation method using a 1.5 MHz subcarrier and a 1 MHz subcarrier are transmitted through packet-communication in which the two packets are transmitted at a cycle of 13.8 ms as is the case with the IrBUS method. As shown in FIGS. 8B and 8C, when the waveform at the time of receiving the packet is analyzed, the waveform may be divided into 1.5 MHz/1 MHz subcarrier component, a base band component and a pattern in which the packet is transmitted. Herein, the base band component generated as the result of superposing the base band component by the packet transmission pattern is called xe2x80x9ca packet transmission base band componentxe2x80x9d. FIGS. 9A and 9B show power spectra at 0 to 2 MHz and 0 to 50 MHz when a packet modulated according to the FSK modulation method is transmitted. FIGS. 10A and 10B show the subcarrier components and the base band components divided from the power spectra shown in FIGS. 9A and 9B, respectively.
As is the case with the continuously output FSK signal, the power spectrum of a packet communication base band component ranges to the low frequency region rather than concentrating only on the DC portion. The sub lobe interferes with the main lobe of the remote control, which significantly lowers the communication efficiency. The sub lobe, which depends on the transmission pattern in the packet communication, is necessarily generated regardless of the modulation method employed as long as a packet communication is performed.
It is an object of the present invention to provide an optical transmission apparatus capable of alleviating mutual interferences with an existing optical transmission apparatus, without having to change an existing optical receiving apparatus.
Another object of the present invention is to provide an optical transmission method which permits mutual interferences with an existing optical transmission apparatus to be alleviated, without having to change an existing optical receiving apparatus.
A digital optical transmission apparatus according to one aspect of the present invention includes a modulation portion which modulates data to be transmitted in an ASK modulation method, and an E/O conversion portion which converts an electrical signal output from the modulation portion into an optical signal having a constant DC level in its base band component.
The E/O conversion portion converts an electrical signal output from the modulation portion into an optical signal having a constant DC level in its base band component, and therefore the power spectrum of the base band component may be reduced, which may reduce mutual interferences with an existing optical transmission apparatus. Since a DC-biased signal or an FSK-biased signal is ignored by an existing optical receiving apparatus, the optical receiving apparatus does not need any change.
A digital optical transmission apparatus according to another aspect of the present invention includes a modulation portion which modulates a packet to be transmitted, a redundancy signal generation portion which attaches a redundancy signal at the start of a packet to be output from the modulation portion for a prescribed time period, and an E/O conversion circuit which gradually increases the power supply voltage and converting an electrical signal output from the modulation portion into an optical signal based on the increased power supply voltage within the time period in which the redundancy signal attached by the redundancy signal generation portion is output.
The E/O conversion circuit gradually increases the power supply voltage within the time period in which the redundancy signal attached by the redundancy signal generation portion is output, and converts an electrical signal output from the modulation portion into an optical signal based on the increased power supply voltage, and therefore the density of the high frequency component of the power spectrum generated by the base band component may be reduced, so that mutual interferences with an existing optical transmission apparatus may be alleviated. Since the redundancy signal is ignored by an existing optical receiving apparatus, the optical receiving apparatus does not need any change.
A digital optical transmission apparatus according to another aspect of the present invention include a modulation portion which modulates a packet to be transmitted, a redundancy signal generation portion which attaches a redundancy signal at the end of a packet to be output from the modulation portion for a prescribed time period, and an E/O conversion circuit which gradually reduces the power supply voltage within a time period in which the redundancy signal attached by the redundancy signal generation portion is output and converts an electrical signal output from the modulation portion based on the reduced power supply-voltage into an optical signal.
The E/O conversion circuit gradually reduces the power supply voltage within a time period in which the redundancy signal attached by the redundancy signal generation portion is output and converts an electrical signal output from the modulation portion into an optical signal based on the reduced power supply voltage; and therefore the density of the high frequency component of the power spectrum generated by the base band component may be reduced, so that mutual interferences with an existing optical transmission apparatus may be alleviated. The redundancy signal is ignored by an existing optical receiving apparatus, and therefore the optical receiving apparatus does not need any change.
A digital optical transmission method according to yet another aspect of the present invention includes the steps of modulating data to be transmitted in an ASK modulation method, and converting the modulated electrical signal into an optical signal having a constant DC level in its base band component.
Since the modulated electrical signal is converted into an optical signal having a constant DC level in its base band component, the power spectrum of the base band component may be reduced, so that mutual interferences with an existing optical transmission apparatus may be alleviated. Since a DC-biased signal or an FSK-biased signal is ignored by an existing optical receiving apparatus, the optical receiving apparatus does not need any change.
A digital optical transmission method according to a still further aspect of the present invention includes the steps of modulating a packet to be transmitted, attaching a redundancy signal at the start of the modulated packet for a prescribed time period, and gradually increasing the power supply voltage within the time period of the attached redundancy signal, thereby converting the modulated electrical signal into an optical signal based on the power supply voltage.
Since the power supply voltage is gradually increased within the time period in which the attached redundancy signal output, and the modulated electrical signal is converted into an optical signal based on the power supply voltage, the density of the high frequency component of the power spectrum generated by the base band component may be reduced, so that mutual interference with an existing optical transmission apparatus may be alleviated. Since the redundancy signal is ignored by an existing optical receiving apparatus, the optical receiving apparatus does not need any change.
A digital optical transmission method according to an additional aspect of the present invention includes the steps of modulating a packet to be transmitted, attaching a redundancy signal at the end of the modulated packet for a prescribed time period, and gradually reducing the power supply voltage within the time period of the attached redundancy signal, and converting the modulated electrical signal into an optical signal based on the power supply voltage.
Since the power supply voltage is gradually reduced within the time period in which the attached redundancy signal is output, and the modulated electrical signal is converted into an optical signal based on the power supply voltage, the density of the high frequency component of the power spectrum generated by the base band component may be reduced, so that mutual interferences with an existing optical transmission apparatus may be alleviated. The redundancy signal is ignored by an existing optical receiving apparatus, and therefore the optical receiving apparatus does not need any change.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.